The identification of the Duchenne muscular dystrophy gene and protein in the late 1980's led to high hopes of rapid translation to rational therapeutics. Early reports of delivering new functional genes to mouse model muscle via gene therapy and stem cell therapy fueled this hope. However, these same studies illuminated the very high hurdles facing human applications. Insufficient therapeutic material (cells, viral vectors), challenges in systemic delivery, and immunological hurdles all remain barriers to demonstration of efficacy of exogenous gene delivery. An alternative approach Is to repair the patient's own gene, and two innovative small molecule approaches have emerged as front-line experimental therapeutics: stop codon read through, and exon skipping. Both approaches are In human clinical trials, and aim to coax dystrophin protein production from mutant genes. In the clinically severe dog model of DMD, the exon-skipping approach is the first therapeutic method that showed improvement of multiple functional outcomes. The proposed CORT is focusing on exon-skipping, the approach that holds promise for the majority of DMD patients. Exon skipping as a drug development program Is highly complex. Patients have different mutations, and drugs must be customized to groups of patients sharing overlapping deletions. Given the high cost of drug development, there is an emerging consensus that exon skipping should achieve drug approval 'as a class'. Three exon-specific drugs should be systematically studied, showing safety and efficacy. The procedures and rules optimized for these three drugs can then be generalized to other, less commonly applicable exon- specific drugs, with reduced regulatory hurdles. The goal of this CORT is to systematically study the three most commonly applicable exon-specific drugs (exons 45,51.53). Project 1 will determine the splicing fidelity and protein function corresponding to the In-frame transcripts generated by the drugs. Project 2 will optimize sequence selection for each exon using multiple experimental systems, and test optimized drugs in a 1 yr pre-clinical efficacy study. Project 3 will carry out the first natural history study of the targeted in-frame deletions (Becker muscular dystrophy);this will permit some prediction of clinical efficacy from production of the relevant semi-functional dystrophin proteins. These three Projects draw upon two research cores: Core B (In vitro and In vivo functional assays), and Core C (Molecular diagnostics and cell banking).